Recently, with the spread of compact cameras and single-use cameras, there have been increased photographing opportunities by the use of a silver halide light sensitive photographic material (hereinafter, also referred simply to a photographic material). As a result, desire for enhancement of photographic performance of silver halide photographic materials has become strong and a higher level of photographic performance is desired. Introduction of Advanced Photo System resulted in an increase of enlarged prints so that development of silver halide grains, aiming at enhanced sensitivity and image quality, has become more important.
Of this development, the most basic and important technique is one for enhancing homogeneity of the grain size distribution, i.e., monodispersity of the silver halide emulsion. As is well recognized in the photographic art, silver halide grains of varying sizes require different optimal conditions for chemical sensitization, so that it is difficult to subject a silver halide emulsion comprised of the grains with both grain sizes, i.e., a polydispersed silver halide emulsion having a broad grain size distribution, to optimal chemical sensitization, often leading to increased fog density or insufficient chemical sensitization. The use of a monodispersed silver halide emulsion makes it easy to achieve optimum chemical sensitization and enables preparation of silver halide emulsions with a high sensitivity and a low fog density. Further,a characteristic curve with high contrast can also be expected.
Silver halide emulsions used in photographic materials can be prepared mainly by the so-called single jet method, in which an aqueous solution of an aqueous soluble silver salt such as silver nitrate is added to a reaction vessel containing a dispersing medium and a halide to allow both salts to react and grow grains, or by the so-called double jet method, in which the aqueous soluble silver salt and the halide are simultaneously added through separate nozzles into a reaction vessel containing a dispersing medium, and then allowed to react and grow grains. In cases where preparation is made by the single jet method, it is rather difficult to control the grain size distribution, the halide distribution within the grain or among the grains, and an internal disorder of the grain. Contrarily, with the double jet method, it is relatively easy to control them, as compared to the single jet method, however, it is limited in reduction of non-uniformity due to variations during reaction, or stagnation. JP-A 2-44335 (herein, the term, JP-A refers to unexamined published Japanese Patent Application) discloses a method of providing a pre-reaction vessel, in which ultra-fine grains as a source of solutes are formed while stirred at high speed and later introducing the thus-formed ultra-fine grains into the reaction vessel. In this method, however, at least the space necessary for stirring and a piping line necessary for introducing the solute-source grains to the reaction vessel from the pre-reaction room are needed, allowing the solute-source grains to grow while being retained therein.
To overcome the problems described above, JP-A 4-139441 discloses a preparation method by means of an apparatus in which a silver salt aqueous solution and a halide aqueous solution are introduced through separate routes to a spiral mixing nozzle to allow the solutions to be mixed and react. In this case, however, it was proved that both reaction solutions were insufficiently mixed, partly due to mixing without using a turbulent flow region, and the aspect ratio of the resulting silver halide grains was insufficient. Further, there was no teaching with respect to the grain size/grain size distribution or photographic performance.
The apparatus used in the invention is a branched type static mixing apparatus in which plural introduction tubes and a discharge tube are linked together. A double structure coaxial nozzle disclosed in JP-A 4-182636, a multiple coaxial nozzle disclosed in JP-A 4-139439 and a dual zone reaction apparatus disclosed in JP-A 8-328177 are distinct in the mixing mode from the apparatus according to the invention. JP-A 8-171156 discloses a preparation method of a silver halide emulsion, in which silver salt and halide solutions are simultaneously introduced to a high speed turbulent flow reaction zone, leading to improvements in the variation or transfer of the scale. It uses a stirring system consisting of a mixing head, which is different in the mixing mode from the apparatus according to the invention.
With regard to techniques of preparing monodispersed silver halide tabular grains, JP-A 1-213637 describes a technique for improving the sensitivity and graininess using monodispersed silver halide grains having two parallel twin planes; and JP-A 5-173268 and 6-202258 discloses the method for preparing a silver halide tabular grain emulsion with a narrow size distribution.
In response to the desire for further enhanced photographic performance of photographic materials on the market, however, there is desired a technique of achieving photographic performance exceeding that obtained by employing various techniques in silver halide tabular grain emulsions described above, in particular, superior photographic performance in the main photographic elements, such as sensitivity and graininess.